Using pythagoras formula:

Hyp\(^2\) = adj\(^2\) + opp\(^2\)

5\(^2\) = opp\(^2\) + 3\(^2\) 

5\(^2\) - 3\(^2\) = adj\(^2\)

4 = adj

length of the chord = 2 * 4 = 8cm